The present invention relates in general to the field of rotary machines, and more particularly to the field of AC motors or generators in which the rotor is excited by permanent magnets.
Such machines are well known. When rotor excitation is performed solely by means of permanent magnets, then the machine can be simplified very significantly, in particular by avoiding any need to provide rotor windings and the associated dispositions, i.e. wipers, wiper-carriers, etc. . . . .
This provides a machine that is less expensive and more reliable.
Unfortunately, in known machines it is necessary to provide one magnet per pole of the rotor and this gives rise to problems of weight and of cost depending largely on the cost of the magnets.
Furthermore, in such machines, various problems arise because it is impossible to control the degree of excitation, and these are set out in particular in the document xe2x80x9cAnalyse des possibilitxc3xa9s de fonctionnement en rxc3xa9gime de dxc3xa9sexcitation des moteurs characterized aimants permanentsxe2x80x9d [Analysis of the possibilities of operating permanent magnet motors under deexcitation conditions] by Multon et al., published in J. Phys. III France 5, 1995, les Editions de Physique 1995.
More precisely, in particular for a motor vehicle alternator, the wide range of possible speeds of rotation and also the wide range of electrical power levels drawn, mean that the motor must be capable of delivering a highly variable level of working flux, i.e. it must be capable of giving rise within itself to relatively high levels of defluxing phenomena.
Furthermore, as shown in particular in the above-mentioned document, permanent magnet machines typically have as many magnets as the rotor has poles, thus making them expensive, heavy, and sometimes difficult to design, and in particular they can have insufficient capacity for defluxing.
Thus, for example, a machine whose rotor possesses successive magnets of alternating polarity must include intermediate magnetic pieces for defluxing purposes between each pair of adjacent magnets, the intermediate magnetic pieces serving to carry defluxing flux and thus lowering nominal performance.
Machines are already known in which the number of magnets possessed by the rotor is less that the number of rotor poles, and typically the number of permanent magnets is equal to half the number of poles.
Nevertheless, those known machines present limitations in that the magnetic flux between the magnets and the poles is poorly controlled, thereby degrading machine efficiency, and in that the flux is poorly adapted to applications of the motor vehicle alternator or alternator-starter type which require high defluxing capacity.
The present invention seeks to mitigate the limitations of the state of the art and to propose an electric machine having permanent excitation in which the rotor is less expensive and simpler to make, and is of reduced weight, while still providing defluxing capacity that is compatible in particular with alternator or alternator-starter type applications, and which also enables flux to be well channeled between the magnets and the poles of the rotor.
The present invention thus provides a rotary electric machine comprising a stator possessing a plurality of slots receiving stator windings, and a rotor including excitation means constituted by main permanent magnets received in a rotor structure to define a plurality of successive north and south poles, the number of main permanent magnets provided being half the number of poles, the machine being characterized in that the rotor structure includes a plurality of barrier zones that are substantially impermeable to magnetic flux and that are disposed relative to the main magnets in such a manner that a substantial portion of the magnetic flux entering into a main magnet comes from at least one south pole adjacent to the magnet and a substantial portion of the magnetic flux leaving a magnet goes towards at least one north pole adjacent to the magnet.
Other preferred but non-limiting features of the machine of the invention are as follows:
the main permanent magnets are disposed essentially radially in the rotor and are suitable for generating magnetic flux all in the same tangential direction, and the poles of the rotor are defined by cavities receiving said main magnets alternating with other cavities that are essentially radial and that define said barrier zones;
said cavities open out into the periphery of the rotor so as to separate the poles from one another;
said cavities are separated in pairs by respective zones of the rotor structure having a minimum section selected to minimize leakage flux while still permitting defluxing of the machine;
said cavities are separated from a central orifice for a rotor shaft by a rotor structure zone having a minimum section selected in such a manner as to minimize leakage flux while permitting defluxing of the machine;
the main permanent magnets are distributed essentially tangentially in the rotor and are suitable for generating magnetic flux all having the same radial direction, and the poles of the rotor are defined firstly in register with said main magnets and secondly in register with the intermediate zones of the rotor structure between said main magnets;
the main magnets are set back from the peripheral surface of the rotor structure;
the radially outer surfaces of the main magnets define a portion of the peripheral surface of the rotor;
the poles of the rotor are separated by slots, and each main magnet extends tangentially substantially between two adjacent slots;
the distance between the bottoms of said slots and the respective associated main magnet is selected to minimize leakage flux while permitting defluxing of the machine;
the rotor structure possesses a plurality of cavities extending between the main magnets and the central region of the rotor and which define said barrier zones in such a manner as to channel the flux between said intermediate zones and the radially inner faces of said main magnets;
the rotor possesses alternating radially-oriented tangential-flux main magnets and tangentially-oriented radial-flux main magnets; and
said cavities defining said barrier zones are filled with a material selected from: air; plastics materials; and non-magnetic metals.
Advantageously, the machine as defined above constitutes a motor vehicle alternator or alternator-starter.